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Re: Cheap DIY Router Challenge !!!

Are you looking at the numbers or the colour and apparent (visual) distortion?
To compare different models / materials you have to look at the numbers. The colour / distortion are not proportional, just to show where things are best / worst.

To actually model this, you will need to use a more representative load. It would be best to actually model the MDF torsion box, as it is not the same as a solid. Rib spacing will have an impact. The smaller the area the force is applied to, the more rib spacing will come in to play.

(Apologies if I have misunderstood your post and you already appreciate these things, but they are common mistakes with FEA).

Re: Cheap DIY Router Challenge !!!

Apologies if I have misunderstood your post and you already appreciate these things, but they are common mistakes with FEA.

I'm not going take offense. Thank you for taking the time.

I supplied max deflection as the main point of comparison for the analysis. The reason for not being too specific is because the model is idealized and not going to be representative of the real world. It should be useful for ballpark comparison of materials under the same idealized conditions (i.e., fixing the material along two edges and then applying a uniform force along the top surface). Hopefully these measurements provide some guidance towards building a structure that meets the overall goals.

Towards that end a major issue is having accurate materials data, an accurate model of the materials, and using the information correctly. Manufactured wood products supply different numbers for perpendicular and parallel properties and that is presenting a major problem for me. I have to change the material depending on the direction of applied force. Makes me feel that there is something I'm missing and the reason why I'm being careful with how I apply load and interpret the results.

A bit confused why you suggest testing a torsion box as that is exactly what the last two models show. And more tellingly, at least to me, is that they show the MDF torsion structure having comparable deflection to steel and aluminum under those idealized conditions. That was a surprise.

Your suggestion of a more representative, smaller area of force is something I intend to pursue as more structure and complexity is added to the model. I assume the holly grail is to build up to a model where force is applied by motors and the spindle ... mostly the spindle. Not there yet.

Last edited by volvox311; 04-10-2016 at 10:31 AM.
Reason: to too two 2

Re: Cheap DIY Router Challenge !!!

Nice job. So I take it that the epoxy must soak a ways into the MDF? That would certainly make it resistant to moisture. With MDF, a 8x8 gantry beam is quite affordable and unsurpassed in rigidity, per $ spent.

In a real CNC, there are about 10 components that contribute to the total (aggregate) deflection-at-the-cutter and the stiffness-at-the-cutter:
1. Spindle and Tool
2. Z Car
3. Z Car Linear Bearings & Rails: the xls calculates this
4. Gantry Car
5. Gantry Car Linear Bearings & Rails: the xls calculates this, too
6. Gantry Tube: usually hard to calculate without a computer model like above
7. Gantry Uprights/Risers/Legs
8. Gantry Feet Bearings & Rails: a strategic design will cause these to have little cutter deflection compared to the other linear bearings.
9. Long Frame Tubes that support the long rails
10. The rest of the Frame, Bed, and Spoilboard
11. Workpiece: hmm, well that's 1 more than 10 components. The spot where the cutter contacts the work piece is actually the reference point for cutter deflections. An equal and opposite force of 100 lb is applied to the workpiece, and it deflects, too, but since it's not part of the machine I assume it is rigid. So we're back to 10 components.

For a target total stiffness-at-cutter, there are some posts in the archives on that. Summarized:
Typical CNC machining center: 50,000 - 150,000 lb/in 404 Not Found
Efficient steel cutting: 60,000 lb/in http://www.cnczone.com/forums/mechan...tml#post916569
Efficient aluminum cutting: 20,000+ lb/in (~1/3 of steel, based on modulus)
Efficient wood cutting: 4,000+ lb/in (~1/5 of aluminum, based on modulus)

I think these metrics are needed for any machine you intend to purchase or build. Without them how exactly do you know what you're getting? What is the best upgrade path? Even this low end competition needs some form of analysis and measurement otherwise what exactly is being built and what is it good for? From my perspective this information and learning process is more valuable than the end result. For my own project I'm currently studying isotropic and orthotropic material models and how they apply to FEA analysis. Hopefully the FEA analysis leads to an estimation of total stiffness-at-cutter.